1. Field of the Invention
The present invention relates generally to air supply control systems for internal combustion engines, and more particularly to a system for controlling air supply effected to an internal combustion engine by a plurality of turbosuperchargers accompanying with the internal combustion engine.
2. Description of the Prior Art
In the field of internal combustion engines employed in vehicles, there has been proposed a so-called sequentially controlled supercharging system in which a couple of turbosuperchargers of primary and secondary are provided for an internal combustion engine and so controlled that only the primary turbosupercharger works for supercharging the engine when intake air mass flow in an intake passage of the engine is relatively small and both the primary and the secondary turbosuperchargers work simultaneously for supercharging the engine when the intake air mass flow is relatively large, as disclosed in, for example, the Japanese patent applications published before examination under publication numbers 56-41417 and 59-160022. In such a system, an exhaust cutoff valve is disposed in a portion of an exhaust passage of the engine through which exhaust gas is applied to a turbine of the secondary turbosupercharger and an intake air cutoff valve is also disposed in a portion of the intake passage of the engine through which air compressed by a compressor of the secondary turbosupercharger is supplied to a combustion chamber of the engine, and each of the exhaust cutoff valve and the intake cutoff valve is controlled to be closed and open so as to cause the primary and secondary turbosuperchargers to work in the aforementioned manner.
In connection with the control in operation of the primary and secondary turbosuperchargers, it has been proposed to put the secondary turbosupercharger in a condition of preliminary rotation before the secondary turbosupercharger commences to work for supercharging the engine for the purpose of suppressing torque shock arising on the engine due to time lag in the starting characteristic of the secondary turbosupercharger. In such a case, it has been usual that the preliminary rotation of the secondary turbosupercharger is caused by such a control as to open the exhaust cutoff valve for supplying the turbine of the secondary turbosupercharger with the exhaust gas under a condition in which an intake air relief valve which is provided in a bypass provided to the intake passage for detouring the secondary turbosupercharger is opened.
However, the preliminary rotation of the secondary turbosupercharger thus caused in the manner proposed previously brings about a disadvantage that the primary turbosupercharger is undesirably reduced in its speed of rotation because the exhaust gas branches through the exhaust cutoff valve which is opened under the condition in which the intake air relief valve is opened and thereby the exhaust gas supplied to a turbine of the primary turbosupercharger is reduced and this compels the preliminary rotation of the secondary turbosupercharger to be conducted insufficiently for a relatively short period of time just before the secondary turbosupercharger commences to work for supercharging the engine. Accordingly, in the case where the preliminary rotation of the secondary turbosupercharger is conducted in the manner proposed previously, it is difficult to put the secondary turbosupercharger in a condition of sufficiently high preliminary rotation before the secondary turbosupercharger commences to work for supercharging the engine, and therefore the torque shock arising on the engine is not sufficiently reduced when the secondary turbosupercharger commences to work for supercharging the engine.
In view of the above, it has been also proposed to provide an exhaust bypass valve disposed at a portion of the exhaust passage of the engine for introducing therethrough a relatively small quantity of exhaust gas to a portion of the exhaust passage downstream to the exhaust cutoff valve from a portion of the exhaust passage upstream to the exhaust cutoff valve, so that the turbine of the secondary turbosupercharger is driven to rotate by the exhaust gas passing through the exhaust bypass valve and thereby the secondary turbosupercharger is subjected to its preliminary rotation under a condition in which the exhaust cutoff valve is in its closed state. In the case where such an exhaust bypass valve as mentioned above is used, the intake air relief valve is caused to close on or before a time point at which the exhaust cutoff valve is opened and the preliminary rotation of the secondary turbosupercharger continues until the intake air relief valve is closed, and therefore the secondary turbosupercharger is put in a condition of sufficiently high preliminary rotation caused by the relatively small exhaust gas passing through the exhaust bypass valve just before the exhaust cutoff valve is opened. When the exhaust cutoff valve is opened, the intake air cutoff valve is also opened and the secondary turbosupercharger under the sufficiently high preliminary rotation commences to work for supercharging the engine under a condition in which the intake air relief valve is closed. Consequently, the torque shock arising on the engine is surely suppressed when the second supercharger commences to work for supercharging the engine.
In the meantime, the internal combustion engine equipped with a turbosupercharger is usually provided with a waste gate valve which is operative to prevent compressed air pressure in an intake passage of the engine from exceeding a predetermined value. The waste gate valve is disposed in a bypass passage detouring a turbine of the turbosupercharger and opened for reducing exhaust gas flowing through the turbine of the turbosupercharger when the compressed air pressure in the intake passage reaches the predetermined value.
In the case of an internal combustion engine to which the sequentially controlled supercharging system wherein the primary and secondary turbosuperchargers are provided as aforementioned and both the exhaust bypass valve and waste gate valve are used for controlling exhaust gas flow supplied to the primary and secondary turbosuperchargers is applied, the exhaust gas flowing through the exhaust bypass valve dose not contribute to increase in compressed air pressure in the intake passage but causes the secondary turbosupercharger to be subjected to its preliminary rotation prior to supercharging operation and therefore the exhaust bypass valve functions substantially to limit the compressed air pressure in the intake passage as well as the waste gate valve when only the primary turbosupercharger works for supercharging the engine and, on the other hand, the compressed air pressure in the intake passage is limited by only the waste gate valve when both the primary and secondary turbosuperchargers work simultaneously for supercharging the engine. This results in a fear that the maximum value of the compressed air pressure in the intake passage limited by the exhaust bypass valve is different from the maximum value of the compressed air pressure in the intake passage limited by the waste gate valve and therefore the compressed air pressure in the intake passage has its maximum value which varies suddenly and undesirably so as to hinder the engine from operating stably when the secondary turbosupercharger commences to work for supercharging the engine in addition to the primary turbosupercharger or the secondary turbosupercharger ceases to work for supercharging the engine.